1. Field of the Invention
This invention relates to liquid treatment and more particularly to controlling the flow of particle-laden water to promote particle removal in water and waste water clarification systems.
2. Description of the Prior Art
Clarifiers are used to remove certain materials or particles from liquid. These materials are generally suspended in the liquid. Clarifiers are generally designed to remove these materials under the force of gravity when the flow of the liquid is substantially reduced, as in a very low flow, or quiescent, zone in the clarifier. Since these materials are generally solid and are said to "settle" out of the liquid, they are referred to as "settleable solids". Such settleable solids may include naturally occurring materials (e.g., clay, silts, sand and dirt), chemical precipitants and biological solids.
Clarifiers are used, for example, in water and waste water treatment plants. In water treatment the water drawn from a water supply has various non-settleable colloidal solids therein. When mixed with chemicals, the colloidal solids and the chemicals agglomerate to form solids. In waste water treatment, the solids include organic solids and biological solids, among other wastes. The solids in water and waste water treatment plants range in size and have densities that are essentially the same as water. The solids having a density somewhat greater than that of water will settle under the force of gravity (and are referred to as settleable solids). The solids having a density equal to that of water will not settle (and are referred to as poorly settleable or non-settleable). Reference herein to "both types of solids" includes both settleable and non-settleable solids. The word "solids" as used herein to describe the present invention refers to such settleable solids unless the context indicates otherwise or unless a distinction is made between settleable solids and poorly settling or non-settleable solids. The word "liquid" as used herein to describe the present invention refers to water and waste water.
Water and waste water have been treated in clarifiers to remove the settleable solids. Loose filter media have been used to remove the non-settleable solids, in an attempt to make the water clear and suitable for use or reuse.
An object of water and waste water clarifiers is to create quiescent zones having very low flow rates to promote maximum settlement of the settleable solids to the bottom of the clarifiers. Clarifiers typically include a large detention basin where the settlement of the solids occurs. Tubes or flat sheets mounted at fixed or variable angles relative to the upper surface of the liquid have been used to form multiple, thin liquid flow paths and create quiescent zones within the detention basin in an attempt to promote settling of solids. Plates have also been mounted within the detention basins at fixed or variable angles relative to the upper surface of the liquid. Fixed plates have been constructed out of relatively heavy single sheets of metal or plastic that are supported on their sides and connected to the sidewalls of self-contained modules positioned within the detention basins. Alternatively the tops of such plates have been fixed and a frame attached to the bottoms of such plates is moved to adjust the angle of the plates relative to the upper surface of the liquid.
The clarifiers are designed in an attempt to have the liquid containing the solids flow upwardly between the plates or tubes at flow rates that allow sufficient time for most of the solids to settle onto the plates or tubes. Ideally the solids then slide down the plates or tubes to the bottom of the detention basin for collection, and clarified liquid flows out of the openings between and at the top of the plates or tubes.
Prior art clarifiers have also used loose, buoyant and non-buoyant filter media to trap both types of solids suspended in the liquid. For example, such media has been between 1.5 and 20 mm in diameter and has been used in the form of loose polypropylene pellets. When the loose pellets are buoyant they have been prevented from flowing with the liquid by screens positioned across the flow path. The flowing liquid urges the loose pellets against the screen to form a bed.
To remove both types of solids from the bed of loose pellets, air scouring systems have been provided. These include pipes mounted at fixed positions below the pellets. After the liquid level in the clarifier has risen to an abnormal level, air flows from the fixed pipes, through the loose pellets, expands the bed of pellets and removes both types of solids from the pellets. Both types of solids are collected in an upper trough that is effective only when the liquid level is abnormally high for cleaning the pellets.
Loose, non-buoyant media has also been used with screens and is cleaned in a similar manner using abnormally high liquid levels.
In other embodiments of loose buoyant filter media, various sized spheres have been loosely provided below screens that prevent the spheres from floating out of the clarifier. The larger spheres have been at the bottom of a layer of such sphere and the relative buoyant forces on the spheres are relied on to maintain a continuous gradation of the sizes of the spheres in the filter.
Other clarifiers have used sand beds as filters. Complex mechanisms have been used to remove part of the sand from the bed, clean it and then return the cleaned sand to the bed.
Prior art clarifiers have also included some type of weir or trough extending across, or along the sides of, the detention basins. Such weirs or troughs are effective to form a relatively thick (e.g., two feet) layer of partially clarified liquid on top of the plates or tubes. The weight of such weirs or troughs necessitates additional support within or surrounding the detention basin, adding to the weight and expense of the clarifiers. In general, the liquid flowing from the tubes or plates into this thick layer is subject to changes in flow rate due to random variations in the upward flow of the liquid through different ones of the quiescent zones of the clarifier. This thick layer is used to diffuse the flow of the liquid from the quiescent zones to the weirs, rendering the flow of the liquid more uniform as the liquid enters the weirs that are at the perimeter of the basin or that extend across the basin.